(a) 
Figure 10. Example "Koralpe": 
straints was made for the example "Koralpe" to see the 
relative difference. The results are given in Table 1. 
Table 1: Computational time for the triangulation. 
  
  
  
  
  
example vectorial raster-based 
(mm:ss) (mm:ss) 
Thalham 2:05 0:51 [401x401] 
Koralpe 21:28(2:59) 1:39(1:38)[493x540] 
remarks 1) The time in () is valid for a 
triangulation without the con- 
straints. 
2) Digits in [] indicate the size 
of array for the QVD construction. 
  
  
  
  
5. CONCLUSIONS 
Techniques from raster data processing can lead to 
quite simple and robust solutions for complicated 
problems in surface modelling. The raster-based 
triangulation improves the computational complexion 
at a great deal in case of a large number of existing 
constraints. The medial axis approach derives geo- 
morphological elements from the given contours and 
thus contributes a lot to the improvement of the DTM 
quality in the case where only contours are available 
for terrain modelling. The combination of the two ap- 
proaches leads to an even satisfactory result. So the 
use of raster techniques for digital terrain modelling 
is promising. 
572 
(b) 
(a) given contours and derived geomorphological elements; (b) contours (thick) 
and intermediate contours (thin) derived from a TIN-DTM. 
REFERENCES 
Arcelli, C., Cordella, L.P., Levialdi, S., 1981: From 
Local Maxima To Connected Skeletons. IEEE Trans- 
actions on Pattern Analysis and Machine Intelligence, 
PAMI-3, No. 2, 134-143. 
Aumann, G., Ebner, H., Tang, L., 1990: Automatic 
Derivation of Skeleton Lines From Digitized Con- 
tours. International Archives of Photogrammetry and 
Remote Sensing, Volume 28, Part 4, 330-337. 
Blum, H., 1967: A Transformation for Extracting New 
Descripters of Shape. Proceedings of Symposium on 
Models for Perception of Speech and Visual Form, 
W. Whaten-Dunn, Ed. Cambrige, MA: M.I.T. Press, 
362-380. 
Borgefors, G., 1986: Distance Transformations In 
Digital Images. Computer Vision, Graphics, and 
Image Processing, 34, 344-371. 
Christensen, A.H J., 1987: Fitting a Triangulation to 
Contours. Proceedings Auto-Carto VII, 57-67. 
Clarke, A.L., Gruen, A., Loon, J.C., 1982: The Appli- 
cation of Contour Data for Generating High Fidelity 
Grid Digital Elevation Models. Proceedings Auto- 
Carto 5, 213-222. 
De Floriani, L., Puppo, E., 1988: Constrained Delau- 
nay Triangulation for Multiresolution Surface De- 
scription. Proceedings of th International 
Conference on Pattern Recognition, Rome, Italy, 566- 
569.